1. Field of the Invention
The present invention relates to a heat transfer recording medium having an image receiving layer formed on a support member, and capable of forming an image by hot-melt ink or sublimating heat transfer ink and thermally adhering the layer onto a recording medium such as paper or plastic, and to a printed product obtained by using this heat transfer recording medium.
2. Description of the Related Art
A sublimation type heat transfer recording method is most often used as a method of recording a face image for personal authentication onto an image display body such as a license, employee's ID card, member's card, or credit card.
In this sublimation type heat transfer recording method, a sublimating ink ribbon formed by coating a support sheet with ink containing a sublimating (or heat flowing) dye such that heat transfer is possible, is overlayed on a recording medium having an image receiving layer made of a thermoplastic resin capable of accepting the sublimating dye. The heat transfer ribbon is selectively heated on the basis of image data by a thermal head or the like, thereby forming a desired image on the recording medium by sublimation type heat transfer recording. It is widely known that color images superior in tone reproduction can be easily recorded by this method. However, this sublimation type heat transfer recording method has the problems related to the durability of a card. For example, an image is easily scratched because it is formed near the face region of thermoplastic resin layer, the dye sublimates again to lower the image density with time, and ultraviolet radiation decomposes the dye to change the tone of color of an image.
A melting type heat transfer recording method is also usable. In this method, a hot-melt ink transfer ribbon formed by coating a support sheet with hot-melt ink in which a coloring pigment or dye is dispersed in a binder such as a resin or wax, is overlayed on a recording medium having an image receiving layer made of a thermoplastic resin capable of accepting the hot-melt ink transfer ribbon. The heat transfer ribbon is selectively heated on the basis of image data by a thermal head or the like, thereby recording a desired image by transferring the binder-containing hot-melt ink onto the recording medium. In this method, inorganic or organic pigments generally having high lightfastness can be selectively used as the coloring material. Also, by improving the resin or wax used as the binder, it is possible to provide images hard to scratch and superior in solvent resistance. In addition, special high-security ink is readily formable by mixing a functional material such as a fluorescent pigment or magnetic substance into the ink. The image receiving layer can be any recording medium provided that the medium has a surface adhesive to the binder. So, the image receiving layer can be chosen from various recording media. As described above, this melting type heat transfer recording method is advantageous for the sublimation type heat transfer method.
Still another method is proposed in which an image is formed on a transparent transfer type image receiving layer formed on a base film by the sublimation heat transfer recording method or the melting type heat transfer recording method described above, and this transfer type image receiving layer on which the image is recorded is thermally transferred onto a recording medium such as paper. In this method, after transferring the transfer type image receiving layer itself can function as a surface protective film, so the mechanical strength of the surface is high. Also, by improving the smoothness of the transfer type image receiving layer surface and thereby increasing the affinity to the ink layer, images excellent in tone reproduction can be formed even by the melting type heat transfer method.
Unfortunately, if a printed product on which a transfer type image receiving layer is formed by the above method is stored for long time periods in contact with a film containing a plastic material e.g., a vinyl chloride resin, such as used in a transparent resin cover, this plastic material moves to the transfer type image receiving layer and is fused to the vinyl chloride resin or the like. If this plastic material is peeled, the transfer type image receiving layer is removed from the final recording medium. Alternatively, if a sublimating ink image is recorded on a printed product, the sublimating dye becomes readily diffusible. This smears the contour of the image or discolors the image.
That is, a transfer type image receiving layer formed by the conventional method cannot be stored for long time periods if a resin film containing a plastic material is overlayed on the layer.
Also, the melting type heat transfer recording method basically performs ink adhesion and uses a dot area modulation tone recording method in which tone recording is performed by changing the sizes of transferred dots. Therefore, the method is very sensitive to the surface unevenness of a recording medium to which an image is to be transferred. If the surface is uneven, inferior transfer occurs to make dot size control impossible, resulting in poor tone reproduction.
Various proposals have been made to solve the above problems. One proposed method uses a recording medium having a porous image receiving layer. In this method, fine pores are formed in an image receiving layer of a recording medium, and hot-melt ink is transferred into these fine pores by permeation. This method can provide images superior in tone reproduction. However, a porous image receiving layer generally has low mechanical strength, so the surface is scratched when brought into contact with various rollers and a convey path in a printing apparatus, resulting in image defects.
In another proposed method, an image is formed on a resin layer obtained by forming a transparent image receiving layer/adhesive layer on a film base, and this image receiving layer/adhesive layer is heated and pressurized to adhere or heated transfer onto a base such as paper or plastic to which the image is to be given. In this method, no fine pores are formed in the image receiving layer, so the mechanical strength of the surface is high. In addition, by improving the smoothness of the resin layer surface and thereby increasing the affinity to the ink layer, images excellent in tone reproduction can be formed even by the melting type heat transfer method.
Unfortunately, the above method has the problem that if a low-softening-temperature resin having high adhesion to paper or plastic is used as the image receiving layer/adhesive layer, the reproducibility of the recording image density becomes unstable under the same recording conditions. This is so because a state (center omission) in which no ink is present in the centers of pixel points constructing a transferred ink image occurs.
To prevent this center omission of each pixel point, a resin having a high softening temperature can be used in the image receiving layer/adhesive layer. However, this lowers the adhesion to the base such as paper or plastic.
As described above, a melting type heat transfer recording image receiving layer formed by the conventional method cannot prevent center omission and ensure sufficient adhesion to paper or plastic at the same time.